Formation of a recent hybrid zone offers insight into the geographic puzzle and maintenance of species boundaries in musk turtles

Speciation is the result of an accumulation of reproductive barriers between populations, but pinpointing the factors involved is often difficult. However, hybrid zones can form when these barriers are not complete, especially when lineages come into contact in intermediate or modified habitats. We examine a hybrid zone between two closely related riverine turtle species, Sternotherus depressus and S. peltifer, and use dual‐digest RAD sequencing to understand how this hybrid zone formed and elucidate genomic patterns of reproductive isolation. First, the geographical extent and timing of formation of the hybrid zone is established to provide context for understanding the role of extrinsic and intrinsic reproductive isolating mechanisms in this system. The strength of selection on taxon‐specific contributions to maintenance of the hybrid zone is then inferred using a Bayesian genomic cline model. These analyses identify a role for selection inhibiting introgression in some genomic regions at one end of the hybrid zone and promoting introgression in many loci at the other. When selective pressures necessary to generate outliers to the genomic cline are considered with the geographical and temporal context of this hybrid zone, we conclude that habitat‐specific selection probably limits introgression from S. depressus to S. peltifer in the direction of river flow. However, selection is mediating rapid, unidirectional introgression from S. peltifer to S. depressus, which is probably facilitated by anthropogenic habitat alteration. These findings indicate a potentially imminent threat of population‐level genomic extinction for an already imperiled species due to ongoing human‐caused habitat alteration.     

THE VARIABLE GENOMIC ARCHITECTURE OF ISOLATION BETWEEN HYBRIDIZING SPECIES OF HOUSE MICE

Studies of the genetics of hybrid zones can provide insight into the genomic architecture of species boundaries. By examining patterns of introgression of multiple loci across a hybrid zone, it may be possible to identify regions of the genome that have experienced selection. Here, we present a comparison of introgression in two replicate transects through the house mouse hybrid zone through central Europe, using data from 41 single nucleotide markers. Using both genomic and geographic clines, we found many differences in patterns of introgression between the two transects, as well as some similarities. We found that many loci may have experienced the effects of selection at linked sites, including selection against hybrid genotypes, as well as positive selection in the form of genotypes introgressed into a foreign genetic background. We also found many positive associations of conspecific alleles among unlinked markers, which could be caused by epistatic interactions. Different patterns of introgression in the two transects highlight the challenge of using hybrid zones to identify genes underlying isolation and raise the possibility that the genetic basis of isolation between these species may be dependent on the local population genetic make-up or the local ecological setting.     

Low intraspecific variation for genomic isolation between hybridizing sunflower species

Barriers to gene flow between species result from selection against foreign linkage blocks in hybrids. When the geographic ranges of taxa meet at multiple locations, the opportunity exists for variation in the genetic architecture of isolating barriers. Hybrid zones between two sunflower species (Helianthus annuus and H. petiolaris) in Nebraska and California exhibited remarkably similar patterns of introgression of mapped molecular markers. Congruence among hybrid zones may result from limited intraspecific variation at loci contributing to isolation and from similar selective effects of alleles in the heterospecific genetic background. The observed consistency of introgression patterns across distantly separated hybrid zones suggests that intrinsic forces predominate in determining hybrid zone dynamics and boundaries between these sunflower species.     

Evidence for strong assortative mating,limited gene flow,and strong differentiation across the blue‐footed/Peruvian booby hybrid zone in northern Peru

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue‐footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue‐footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue‐footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue‐footed to Peruvian boobies in this hybrid system.     

ECOLOGICAL SEGREGATION IN A SMALL MAMMAL HYBRID ZONE: HABITAT‐SPECIFIC MATING OPPORTUNITIES AND SELECTION AGAINST HYBRIDS RESTRICT GENE FLOW ON A FINE SPATIAL SCALE

The degree to which closely related species interbreed is determined by a complex interaction of ecological, behavioral, and genetic factors. We examine the degree of interbreeding between two woodrat species, Neotoma bryanti and N. lepida, at a sharp ecological transition. We identify the ecological association of each genotypic class, assess the opportunity for mating between these groups, and test whether they have similar patterns of year‐to‐year persistence on our study site. We find that 13% of individuals have a hybrid signature but that the two parental populations and backcrosses are highly segregated by habitat type and use. Also, we find that adult hybrids are comparable to parental types in terms of year‐to‐year persistence on our site but that, among juveniles, significantly fewer hybrids reach adulthood on site compared to their purebred counterparts. Our analyses show that this hybrid zone is maintained by occasional nonassortative mating coupled with hybrid fertility, but that these factors are balanced by lower apparent survival of juvenile hybrids and habitat‐based preference or selection that limits heterospecific mating while promoting backcrossing to habitat‐specific genotypes. This system presents a novel example of the role that sharp resource gradients play in reproductive isolation and the potential for genetic introgression.     

Nuclear markers,mitochondrial DNA and male secondary sexual traits variation in a newt hybrid zone (Triturus vulgaris x T. montandoni)

The smooth and the Montandon's newts (Triturus vulgaris and T. montandoni) are genetically similar sister species with highly divergent male secondary sexual traits involved in complex courtship behaviour. Their parapatric ranges overlap at moderate elevations in the Carpathian Mountains where they hybridize readily. Here we present a detailed study of genetic and morphological variation in populations from the area of sympatry. Analysis of variation at seven nuclear markers, mtDNA and male sexual secondary traits was complemented with an ecological survey of breeding sites characteristics. Extensive hybridization was revealed with back-cross individuals similar to either parental species predominating among hybrids. The hybrid zone exhibited a mosaic pattern: the genetic composition of the populations was correlated only weakly with their geographical position. No association with habitat type was found. Departures from Hardy-Weinberg proportions, significant linkage disequilibria and bimodal distribution of genotypes suggest strongly that assortative mating is an important factor shaping the genetic composition of hybrid populations. The pattern of cytonuclear disequilibria did not indicate much asymmetry in interspecific matings. Changes in the frequency of nuclear markers were highly concordant, whereas mtDNA showed much wider bidirectional introgression with 14% excess of T. montandoni haplotype. We argue that the mosaic structure of the newt hybrid zone results mainly from stochastic processes related to extinction and recolonization. Microgeographical differences in mtDNA introgression are explained by historical range shifts. Since morphologically intermediate males were underrepresented when compared to hybrid males identified by genetic markers, sexual selection acting against the morphological intermediates is implied. We discuss the implications of these findings in the context of reinforcement of prezygotic isolation in newts.     

ECOLOGICAL GENETICS OF A MOSAIC HYBRID ZONE: MITOCHONDRIAL,NUCLEAR, AND REPRODUCTIVE DIFFERENTIATION OF CRICKETS BY SOIL TYPE

We investigated the effects that habitat variation has on the structure and dynamics of a hybrid zone between two closely related crickets in Connecticut. A collecting protocol was developed in which crickets were sampled from characteristic habitats on either side of the hybrid zone and from two distinct habitat types within the zone. Presumptive pure Gryllus pennsylvanicus were sampled from fields in northwestern Connecticut and represent “inland” populations. “Pure” Gryllus firmus were sampled from beaches along the coast and represent the “coastal” populations. Crickets from within the hybrid zone were sampled from two different soil types: the “loam” populations from loamy soils and the “sand” populations from sandy soils. Moreover, an attempt was made to identify closely adjacent sand and loam localities to determine the scale of habitat variation and its possible effects on hybrid-zone structure. In general, there was little variation in morphological traits or in allozyme and mtDNA genotype frequencies among localities from within each of the four habitat types. Between each of the closely situated sand and loam localities within the hybrid zone, however, there were very significant differences in each of these sets of markers. In addition, crickets from hybrid-zone populations were tested for reproductive isolation. The asymmetric outcome of hybrid crosses that exists across the zone (Harrison, 1983) also exists on a finer ecological scale within the zone. Thus, this hybrid zone is a mosaic of strikingly differentiated populations. The dynamics of hybrid zones with mosaic structures are discussed in contrast to the traditional clinal models. The data are also discussed in light of the semipermeable nature of species boundaries. The extent to which a species boundary is permeable varies not only from one genetic marker to the next, but also with the ecological and geographic context of species interaction.     

The modality of nine Triturus newt hybrid zones assessed with nuclear,mitochondrial and morphological data

The shape of hybrid zones provides insight into genetic isolation between the evolutionary lineages involved: the greater the limitation to gene flow, the farther along the unimodal to bimodal continuum. We study hybrid zone modality in a group of closely related species at a variety of levels of genetic divergence. We explore the degree and the geography of hybridization in 12 transects of Triturus newts, for nine of the ten secondary contact zones in Europe, using nuclear and mitochondrial encoded genetic data and morphological characteristics. Species status is verified by examining the correlation between the three marker systems in a geographical context. At 40 allozyme loci, two transects classify as unimodal, three as bimodal and two as intermediate, while the signal for hybridization is weak or absent in five transects. One zone studied in duplicate was classified as intermediate in one region and as bimodal in another region. mtDNA introgression is frequent and extends beyond nuclear introgression in two transects. Morphology provides additional evidence for hybridity, including one transect for which the signal of nuclear gene flow is weak. Compared to simulations allowing panmixia, the observed allozyme transitions at contact zones show a deficit of backcrossing to various degrees. Over all transects, there is a weak negative relationship between the level of hybridization and allozyme genetic distance for species pairs, consistent with Bateson–Dobzhansky–Muller effects. This observation, based upon highly comparable data for a single genus, supports conclusions derived from analyses over a wide variety of other taxa. © 2014 The Authors. Biological Journal of the Linnean Society published by John Wiley & Sons Ltd on behalf of The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 604–622.     

Gene flow in Dubautia arborea and D. ciliolata: the roles of ecology and isolation by distance in maintaining species boundaries despite ongoing hybridization

The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently 'pure' populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat.     

Genetic structure of natural populations of Castanea sativa in Turkey: evidence of a hybrid zone

This study points out the evidence of a hybrid zone between two groups of genetically differentiated populations of chestnut (Castanea sativa Mill.) in Turkey. Genetic structure, gene flow and introgression levels, based on 16 allozyme loci, were investigated on 34 population samples spanning the entire C. sativa distribution area in this country. The occurrence of the hybrid zone, located in the Bithynian region, was inferred in a group of populations showing the following genetic characteristics: (i) enhanced genetic variability and intermediate allelic frequencies between those of the Western and Eastern groups of populations; (ii) sharp and concordant changes in allele frequencies; (iii) decreased gene flow with the Western and Eastern populations. Starting from the cline width estimated to be 324 km, strength of selection was evaluated from the gene flow distance, as indicated from the degree of genetic structuring outside the hybrid zone. Evolutionary processes shaping the observed genetic differentiation and introgression are discussed on the basis of palynological data, palaeoclimatic events and evidence of hybridization found in other plant and animal species in the same region.     

Discordant patterns of introgression across a narrow hybrid zone between two cryptic lineages of an Iberian endemic newt

The study of natural hybrid zones can illuminate aspects of lineage divergence and speciation in morphologically cryptic taxa. We studied a hybrid zone between two highly divergent but morphologically similar lineages (south‐western and south‐eastern) of the Iberian endemic Bosca's newt (Lissotriton boscai) in SW Iberia with a multilocus dataset (microsatellites, nuclear and mitochondrial genes). STRUCTURE and NEWHYBRIDS analyses retrieved few admixed individuals, which classified as backcrosses involving parental individuals of the south‐western lineage. Our results show asymmetric introgression of mtDNA beyond the contact from this lineage into the south‐eastern lineage. Analysis of nongeographic introgression patterns revealed asymmetries in the direction of introgression, but except for mtDNA, we did not find evidence for nonconcordant introgression patterns across nuclear loci. Analysis of a 150‐km transect across the hybrid zone showed broadly coincident cline widths (ca. 3.2–27.9 km), and concordant cline centres across all markers, except for mtDNA that is displaced ca. 60 km northward. Results from ecological niche modelling show that the hybrid zone is in a climatically homogenous area with suitable habitat for the species, suggesting that contact between the two lineages is unlikely to occur further south as their distributions are currently separated by an extensive area of unfavourable habitat. Taken together, our findings suggest the genetic structure of this hybrid zone results from the interplay of historical (biogeographic) and population‐level processes. The narrowness and coincidence of genetic clines can be explained by weak selection against hybrids and reflect a degree of reproductive isolation that is consistent with cryptic speciation.     

Evolution of a hybrid zone of two willow species (Salix L.) in the European Alps analyzed by RAD‐seq and morphometrics

Natural hybridization of plants can result in many outcomes with several evolutionary consequences, such as hybrid speciation and introgression. Natural hybrid zones can arise in mountain systems as a result of fluctuating climate during the exchange of glacial and interglacial periods, where species retract and expand their territories, resulting in secondary contacts. Willows are a large genus of woody plants with an immense capability of interspecific crossing. In this study, the sympatric area of two diploid sister species, S. foetida and S. waldsteiniana in the eastern European Alps, was investigated to study the genomic structure of populations within and outside their contact zone and to analyze congruence of morphological phenotypes with genetic data. Eleven populations of the two species were sampled across the Alps and examined using phylogenetic network and population genetic structure analyses of RAD Seq data and morphometric analyses of leaves. The results showed that a homoploid hybrid zone between the two species was established within their sympatric area. Patterns of genetic admixture in homoploid hybrids indicated introgression with asymmetric backcrossing to not only one of the parental species but also one hybrid population forming a separate lineage. The lack of F1 hybrids indicated a long‐term persistence of the hybrid populations. Insignificant isolation by distance suggests that gene flow can act over large geographical scales. Morphometric characteristics of hybrids supported the molecular data and clearly separated populations of the parental species, but showed intermediacy in the hybrid zone populations with a bias toward S. waldsteiniana. The homoploid hybrid zone might have been established via secondary contact hybridization, and its establishment was fostered by the low genetic divergence of parental species and a lack of strong intrinsic crossing barriers. Incomplete ecological separation and the ability of long‐distance dispersal of willows could have contributed to the spatial expansion of the hybrid zone.     

Evolution of a hybrid zone of two willow species (Salix L.) in the European Alps analyzed by RAD-seq and morphometrics

Natural hybridization of plants can result in many outcomes with several evolutionary consequences, such as hybrid speciation and introgression. Natural hybrid zones can arise in mountain systems as a result of fluctuating climate during the exchange of glacial and interglacial periods, where species retract and expand their territories, resulting in secondary contacts. Willows are a large genus of woody plants with an immense capability of interspecific crossing. In this study, the sympatric area of two diploid sister species, S. foetida and S. waldsteiniana in the eastern European Alps, was investigated to study the genomic structure of populations within and outside their contact zone and to analyze congruence of morphological phenotypes with genetic data. Eleven populations of the two species were sampled across the Alps and examined using phylogenetic network and population genetic structure analyses of RAD Seq data and morphometric analyses of leaves. The results showed that a homoploid hybrid zone between the two species was established within their sympatric area. Patterns of genetic admixture in homoploid hybrids indicated introgression with asymmetric backcrossing to not only one of the parental species but also one hybrid population forming a separate lineage. The lack of F1 hybrids indicated a long-term persistence of the hybrid populations. Insignificant isolation by distance suggests that gene flow can act over large geographical scales. Morphometric characteristics of hybrids supported the molecular data and clearly separated populations of the parental species, but showed intermediacy in the hybrid zone populations with a bias toward S. waldsteiniana. The homoploid hybrid zone might have been established via secondary contact hybridization, and its establishment was fostered by the low genetic divergence of parental species and a lack of strong intrinsic crossing barriers. Incomplete ecological separation and the ability of long-distance dispersal of willows could have contributed to the spatial expansion of the hybrid zone.     

WEAK CROSSABILITY BARRIER BUT STRONG JUVENILE SELECTION SUPPORTS ECOLOGICAL SPECIATION OF THE HYBRID PINE PINUS DENSATA ON THE TIBETAN PLATEAU

Determining how a new hybrid lineage can achieve reproductive isolation is a key to understanding the process and mechanisms of homoploid hybrid speciation. Here, we evaluated the degree and nature of reproductive isolation between the ecologically successful hybrid species Pinus densata and its parental species P. tabuliformis and P. yunnanensis. We performed interspecific crosses among the three species to assess their crossability. We then conducted reciprocal transplantation experiments to evaluate their fitness differentiation, and to examine how natural populations representing different directions of introgression differ in adaptation. The crossing experiments revealed weak genetic barriers among the species. The transplantation trials showed manifest evidence of local adaptation as the three species all performed best in their native habitats. Pinus densata populations from the western edge of its distribution have evolved a strong local adaptation to the specific habitat in that range; populations representing different directions of introgressants with the two parental species all showed fitness disadvantages in this P. densata habitat. These observations illustrate that premating isolation through selection against immigrants from other habitat types or postzygotic isolation through selection against backcrosses between the three species is strong. Thus, ecological selection in combination with endogenous components and geographic isolation has likely played a significant role in the speciation of P. densata.     

Differential patterns of introgression across the X chromosome in a hybrid zone between two species of house mice

A complete understanding of the speciation process requires the identification of genomic regions and genes that confer reproductive barriers between species. Empirical and theoretical research has revealed two important patterns in the evolution of reproductive isolation in animals: isolation typically arises as a result of disrupted epistatic interactions between multiple loci and these disruptions map disproportionately to the X chromosome. These patterns suggest that a targeted examination of natural gene flow between closely related species at X-linked markers with known positions would provide insight into the genetic basis of speciation. We take advantage of the existence of genomic data and a well-documented European zone of hybridization between two species of house mice, Mus domesticus and M. musculus, to conduct such a survey. We evaluate patterns of introgression across the hybrid zone for 13 diagnostic X-linked loci with known chromosomal positions using a maximum likelihood model. Interlocus comparisons clearly identify one locus with reduced introgression across the center of the hybrid zone, pinpointing a candidate region for reproductive isolation. Results also reveal one locus with high frequencies of M. domesticus alleles in populations on the M. musculus side of the zone, suggesting the possibility that positive selection may act to drive the spread of alleles from one species on to the genomic background of the other species. Finally, cline width and cline center are strongly positively correlated across the X chromosome, indicating that gene flow of the X chromosome may be asymmetrical. This study highlights the utility of natural populations of hybrids for mapping speciation genes and suggests that the middle of the X chromosome may be important for reproductive isolation between species of house mice.     

Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation‐in‐action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs Pseudacris feriarum and P. nigrita to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.     

Assessing multilocus introgression patterns: a case study on the mouse X chromosome in central Europe

Multilocus hybrid zone (HZ) studies predate genomics by decades. The power of early methods is becoming apparent and now large datasets are commonplace. Relating introgression along a chromosome to evolutionary process is challenging: although reduced introgression regions may indicate speciation genes, this pattern may be obscured by asymmetric introgression of linked invasive genes. Further, HZ movement may form salients and leave islands in its wake. Barton's concordance was proposed 24 years ago for assessing introgression where geographic patterns are complex. The geographic axis of introgression is replaced with the hybrid index. We compare this, a recently proposed genomic clines approach, and two-dimensional (2D) geographic analyses, for 24 X chromosome loci of 2873 mice from the central-European house mouse HZ. In 2D, 14 loci show linear contact, seven precisely matching previous studies. Four show introgression islands to the east of the zone, suggesting past westward zone movement, two show westward salients. Barton's concordance both recovers and refines this information. A region of reduced introgression on the central X is supported, despite X centromere-proximal male-biased westward introgression matching a westward 2D geographic salient. Genomic clines results are consistent regarding introgression asymmetries, but otherwise more difficult to interpret. Evidence for genetic conflict is discussed.     

Populations composed entirely of hybrid colonies: bidirectional hybridization and polyandry in harvester ants

In eusocial Hymenoptera, haplodiploid life cycles, obligate sterile castes, and polyandry may facilitate selection for hybridization. We analyzed a broad hybrid zone between the ecologically distinct seed‐harvester ants Pogonomyrmex occidentalis (Cresson) and Pogonomyrmex maricopa (Wheeler) using mitochondrial (mt)DNA sequence data, eight morphological markers, and 14 random amplified polymorphic DNA (RAPD) markers. Average mtDNA sequence divergence among parental species was 11.34%, indicating secondary contact. RAPD markers were significantly correlated with morphological variation, confirming the interspecific hybrid origin of all morphologically putative hybrid colonies. A morphological hybrid index indicates an abundance of both F₁ hybrids and parental morphotypes within colonies. Individual character frequencies plotted against distance show coincident and concordant clines, suggesting little to no introgression. The structure of the hybrid zone is two‐fold. Within the western region, stark reversals in character frequencies coincide with overt soil differences, indicating a mosaic hybrid zone structure. The eastern region is a riparian habitat where four adjacent populations were composed entirely of hybrid colonies. These habitat associations suggest that hybrid worker genomes permit dispersal into intermediate environments that select against one or both parental species. The present study suggests that, in addition to retaining reproductive compatibility, ecologically distinct species of ants may generate hybrid colonies maintained by environmental selection. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 320–336.     

STRUCTURE OF AN ASYMMETRIC HYBRID ZONE BETWEEN TWO WATER STRIDER SPECIES (HEMIPTERA: GERRIDAE: LIMNOPORUS)

The water stricter species Limnoporus dissortis and L. notabilis hybridize across a broad zone in western Canada. Body length and alleles at four allozyme loci show a steep cline along the east slope of the Rocky Mountains in western Alberta, while in central British Columbia the parental phenotypes coexist without merging fully. One sex-linked locus shows little introgression, while there is apparently considerable gene flow at three autosomal loci. Although the hybrid zone has characteristics of a broad tension zone, the spatial distribution of introgression suggests that habitat patchiness and differential habitat associations of the two species also contribute to the pattern of hybridization. Asymmetry in interspecific mating success and incompatibilities of sex chromosomes with each other or with cytoplasmic factors appear to account for the occurrence of L. dissortis genotypes within the range of L. notabilis, and the lack of L. notabilis genotypes within the range of L. dissortis. The genetic structure of this hybrid zone supports the importance of sex-linked traits in maintaining the integrity of species, while its spatial structure suggests that extrinsic habitat features can combine with intrinsic genetic incompatibilities to produce complex hybrid interactions.